(1) Field of the Invention
The present invention relates to a wavelength demultiplexing unit suitable for use in an optical system for switching a transmission channel for each wavelength component (hereinafter referred to as a wavelength selection switch) employed in, for example, a WDM (Wavelength Division Multiplexing) transmission system.
(2) Description of Related Art
Recently, in order to handle traffic data quantity dramatically increasing in the Internet, rapid development can be observed in an optical-technology oriented network which seeks an optical arrangement (i.e., optical communication or optical interface) having a wavelength division multiplexing communication as a core technology. The current network topology (network of a WDM transmission system) is, for example, a point-to-point type in which a couple of terminal nodes (transmission node apparatus) are directly connected to each other. However, prediction about network development is made such that a network topology in the near future will become a ring-type or a mesh type (see Non-Patent Document 1).
Further, each of the nodes constituting the network is expected to carry out switching directly on the optical signals concerning the Add and Drop processing for demultiplexing and inserting a signal light having a desired wavelength without doing OE conversion (Optical to Electrical conversion) between an optical signal and electrical packet (electrical-signal), EO conversion (Electrical to Optical conversion) and the like. With this operation, each node can perform processing such as optical signal channel switching or the like at a high speed. As a consequence, channel setting and channel change can be performed dynamically.
The above-described optical switch can change independently a transmission channel of a single wavelength light contained in the wavelength division multiplexing light inputted into the optical switch, and n-single wavelength lights having the transmission channel changed are again subjected to the wavelength division multiplexing depending on necessity. That is, the optical switch selects one or a plural number of single wavelength lights from the n-single wavelength lights, synthesizes the selected one or a plural number of single wavelength lights to form a wavelength division multiplexing signal light, and outputs the wavelength division multiplexing signal light.
Meanwhile, wavelengths of respective single wavelength lights are based on a specification known as ITU grid which is standardized by ITU (International Telecommunication Union). Therefore, the number of wavelength division multiplexing employed in the photonic network system is determined based on the regulation.
(X1) Network Arrangement
Next, description will be made on a case in which the wavelength selection switch is provided in a mesh type network.
FIG. 11 is a diagram for explaining a wavelength selection switch provided in a mesh type network system. As shown in FIG. 11, a network 100 is a mesh type one. Further, in FIG. 11, reference numerals 201 to 204 represent wavelength selection switches, 205 a multiplexer, 206 and 207 input ports of the wavelength selection switch 203, and 208 and 209 output ports.
The wavelength selection switch 203 will be described as one for carrying out switching among eight single wavelength lights (light rays) in an optical, manner, for example. A wavelength division multiplexed signal light (W1) transmitted from the wavelength selection switch 201 and a wavelength division multiplexed signal light (W2) transmitted from the multiplexer 205 are supplied to the wavelength selection switch 203 at the input ports 206 and 207, respectively. The wavelength selection switch 203 carries out switching operation independently on transmission channels for the eight single wavelength lights in total, i.e., four single wavelength lights contained in the wavelength division multiplexed signal light (W1) and four single wavelength lights contained in the wavelength division multiplexed signal light (W2). Thus, the eight single wavelength lights having a transmission channel changed by the switching operation are subjected to wavelength division multiplexing to form wavelength division multiplexed signal lights (W3, W4). Each of the created wavelength division multiplexed signal lights (W3, W4) are outputted from the output ports 208 and 209, respectively.
(X2) Wavelength Selection Switch
FIG. 12 is a diagram for illustrating a case in which a 2×2 switch (2×2 SW) is employed as a wavelength selection switch 203. As shown in FIG. 12, reference numerals 204a and 204b represent demultiplexing units, 205 a 2×2 switch, 204c and 204d multiplexing units, respectively. As the 2×2 switch, MEMS (Micro Electro Mechanical Systems) can be employed, and a reflection type diffraction grating can be employed as a device serving as the wavelength demultiplexing unit and the wavelength multiplexing unit.
Operation of the wavelength selection switch 203 will be described.
A wavelength division multiplexed signal light W1 (containing signal lights of single wavelengths, λ2, λ5, λ6) and a wavelength division multiplexed signal light W2 (containing signal lights of single wavelengths, λ1, λ3, λ4, λ7) are subjected to demultiplexing operation in the demultiplexing units 204a and 204b, respectively. The demultiplexed lights are inputted into the 2×2 switches 205 provided for the single wavelength lights, respectively. The 2×2 switches 205 determines which of the inputted single wavelength signal lights shall be outputted from which of the output units. A single wavelength light outputted from a first output port (an upper side output port in FIG. 12) of the 2×2 switch 205 is inputted into the wavelength synthesizer 204c and a single wavelength light outputted from a second output port (an lower side output port in FIG. 12) of the 2×2 switch 205 is inputted into the wave synthesizer 204d. If the 2×2 switch 205 has an attenuation function, the degree of attenuation may be adjusted upon the selection so that the respective single wavelength lights come to have substantially the same power when the lights are applied to the wave synthesizing units 204c and 204d. In this way, it becomes possible to control each of the wavelength signal lights to have a substantially uniform level of power by a single amplifier (not shown) after the signal lights are synthesized in the wave synthesizers 204c and 204d. 
The wavelength division signal lights (W3 and W4) deriving from the wavelength synthesis in the wavelength synthesizers 204c and 204d are outputted from the output ports 208 and 209.
(X3) Wavelength Selection Switch having Monitoring Function
What set forth above is an explanation of the main function of the wavelength selection switch 203. Now description will be more fully made on control of the 2×2 switch 205 in relation with the monitoring of the signal lights having undergone the wavelength selection.
FIG. 13 is a diagram illustrating an arrangement of a conventional wavelength selection switch having a monitoring function.
In FIG. 13, reference numerals 210a and 210b represent circulators, 211a and 211b optical couplers for demultiplexing an optical signal, 212a and 212b spectrum monitors for observing spectrum of the inputted lights, 213 a control circuit for controlling an angle of a mirror of the 2×2 switch (e.g., MEMS), 214a to 214d driving circuits (drivers) for outputting driving voltages for controlling the angle of MEMS mirrors corresponding to respective wavelengths (in this case, four wavelengths), 215 a collimator lens, 216 a demultiplexing section for implementing the functions of the demultiplexing units 204a and 204b (FIG. 12) and the wave synthesizers 204c and 204d, 217 an input/output optical system, 218a and 218b a first output unit and a second input/output unit, respectively.
Operation of the above arrangement will be briefly described.
As will be described above, the wavelength division multiplexed signal lights (W1, W2) are inputted to the input ports 206 and 207 owing to optical fiber connection. Each of the wavelength division multiplexed signal lights (W1, W2) is supplied to the first input/output unit 201a and the second input output unit 218b through the circulators 210a and 210b. The inputted optical signals are converted into collimated lights by the input/output optical system 217. That is, the input/output optical system 217 has collimators provided at the tip end of respective optical fibers so that inputted lights can be collimated.
The WDM signals (W1 and W2) shaped into collimated lights are irradiated onto different spots (spots shifted from each other in Z-direction) on the diffraction grating as the demultiplexing section.
Lights having respective wavelengths deriving from the demultiplexing are collimated by the collimator lens 215 and incident on respective MEMS mirrors constituting the 2×2 switch 205. In this case, illustration is made for easy understanding in such a manner that four MEMS mirrors are provided and the wavelength division multiplexed signals W1 and W2 share no common signal component having the same wavelength.
The single wavelength lights irradiated onto the 2×2 switch 205 (MEMS) is reflected on the corresponding MEMS mirrors and again irradiated onto the demultiplexing section 216 through the collimator lens 215.
Now, description will be made on a case in which the single wavelength light (λ1) contained in the wavelength division multiplexed signal (W1) inputted at the input port 206 is outputted at the output port 209, with reference to FIG. 4(a). As shown in FIG. 4(a), the wavelength division multiplexed signal (W1) inputted at the input port 206 is fed to a first input/output unit 31a through a circulator 11a, and subjected to the demultiplexing in the demultiplexing section 5. The single wavelength light having the wavelength of λ1 is irradiated onto the 2×2 switch 205 (an MEMS mirror 81 for the single wavelength light having the wavelength of λ1) through the collimator lens 6.
The single wavelength component of λ1 is reflected upon the MEMS mirror 81 which is controlled in its angle in accordance with a control voltage outputted from the driving circuit 214a to 214d (FIG. 13) operated based on the control of the control circuit 213. The single wavelength component of λ1 is irradiated onto the demultiplexing section 5 (FIG. 4(a)) through the collimator lens 6. At this time, the single wavelength component of λ1 is led to the demultiplexing section 5 to form a spot thereon which corresponds to a position leading the component to the second input/output unit 31b owing to the reflection on the demultiplexing section 5. Therefore, the MEMS mirror 81 is controlled in its angle so that the light is returned to the optical fiber connected to the second input/output unit 31b. 
Meanwhile, description will be made on a case in which the single wavelength light (λ1) contained in the wavelength division multiplexed signal (W1) inputted at the input port 206 is outputted at the output port 208, with reference to FIG. 4(b).
As shown in FIG. 4(b), the wavelength division multiplexed signal (W1) inputted at the input port 206 is fed to a first input/output unit 31a through a circulator 11a, and subjected to the demultiplexing in the demultiplexing section 5. The single wavelength light having the wavelength (λ1) is irradiated onto the 2×2 switch 205 (an MEMS mirror 81 for the single wavelength light having the wavelength of λ1) through the collimator lens 6.
The single wavelength component (λ1) is reflected upon the MEMS mirror 81 which is controlled in its angle in accordance with the control voltage (or voltages) outputted from the driving circuit 214a to 214d (see FIG. 13) operated based on the control of the control circuit 213. The single wavelength component is irradiated through the collimator lens 6 (FIG. 4(a)) onto the demultiplexing section 5 at the spot which is substantially the same as that to which the single wavelength component is first incident thereon. That is, single wavelength component is led to the spot which corresponds to a position leading the component to the first input/output unit 31a. Therefore, the MEMS mirror 81 is controlled in its angle so that the light is returned to the same optical fiber connected to the first input/output unit 31a. 
As described above, the optical signal reflected on the 2×2 switch 205 is outputted from one of the first input/output unit 31a and the second input/output unit 31b depending on the reflection angle, and led to the output port 208 or the 209 through the circulator 11a or 11b. 
As described above, transmission channels can be changed for each of the optical signals having respective wavelengths in a desired manner in accordance with the angle control of the 2×2 switch 205. However, the angle control is so delicate that it is requested to monitor whether proper angle setting is achieved or not for optimizing the state of control.
(X4) Control on 2×2 Switch by Monitor
One of prior arts handles the above-described problems in a manner as follows. That is, as shown in FIG. 13, the spectrum monitors 212a and 212b are provided for performing monitoring on the optical signals having undergone the wavelength selection by demultiplexing with the couplers 211a and 211b. That is, the signal lights of respective wavelengths are measured in their power and the result thereof is supplied to the control circuit 213.
The control circuit 213 controls the driving circuits 214a to 214d corresponding to the MEMS mirrors assigned to respective wavelengths based on the results of measurement on respective wavelengths so that the driving voltage is (or voltages are) adjusted and so-called feedback control is effected so that the MEMS mirrors are set to have optimized angles.
When the optimized angle is determined for the MEMS mirrors, the manner of determining the angle seeks not only one making it possible to minimize the coupling loss thereof but also so-called VOA (Variable Optical Attenuator) function realization for variably determining the amount of attenuation in the optical power of the single wavelength light. In other words, the angle of the MEMS mirror may be deviated from an angle making the coupling loss to be minimized, or the angle is intentionally set to an angle making the loss of the single wavelength light be increased.
While several examples of the prior art have been described above, plenty of technologies relating to the wavelength selection have been proposed. In particular, Patent Document 1, for example, discloses a WSR (Wavelength-Separating-Routing) apparatus having a monitoring function which performs wavelength division demultiplexing on a signal by using a diffraction grating. Non-Patent Document 2 discloses an apparatus employing a VIPA (Virtually Imaged Phase Array) as a wavelength demultiplexing element. Further, Non-Patent Document 3 discloses an apparatus using a demultiplexing unit which is fabricated by using a photonic crystallization technology.
Patent Document 1: U.S. Pat. No. 6,549,699
Non-Patent Document 1: Monthly Bulletin of Institute of Electronics, Information and Communication Engineers, February 2002, pp. 94-103.
Non-Patent Document 2: M. Shirasaki “Large angular dispersion by a virtually imaged phased array and its application to a wavelength demultiplexer”, OSA Optics Letters, Vol. 21, No. 5, pp. 366-368
Non-Patent Document 3: Monthly Bulletin of “Optronics” (published by The Optronics Co., Ltd.) July 2001, pp. 179-208.
However, according to the prior art technology, it is necessary to provide a spectrum monitor separately from the wavelength selection switch for feedback control. For this reason, the prior art technology encounters difficulty in small-sizing the wavelength selection switch.